If air is taken as a binary solution, the solvent is                        

Air is a mixture of components${\text{(N}}_{\text{2}}\text{,}{\text{O}}_{\text{2}}\text{,}{\text{CO}}_{\text{2}}\text{,}\text{Ar,}{\text{H}}_{\text{2}}\text{and}\text{Ne)}$ .  If we consider total air as a binary solution, then the solvent is nitrogen ${\text{(N}}_2)$  because it has large quantity.